I'm not so sure...
Nomex is a a made aramid fiber that is inherently flame resistant, it has no treatments that will degrade, wash out or otherwise be affected by exposure to the elements. Nomex will fade with exposure to UV, however Dupont says this fading does not affect it's effectiveness. There is no set number of times Nomex may be exposed to heat before needing to be replaced, expose my station wear or turnouts to high enough heat once and they can be trashed. More likely an individual may only need 3 sets of turnouts in a 25 year career. Some guys physically wear the gear out with no apparent damage from the heat from the fires they've been in. Generally Nomex fabric is extremely durable and wears like iron. While the Nomex uniform I wear to work costs several times as much as the FR treated cotton alternative, it lasts several years and the fire resistance is permenant as opposed to several months with the treatment washing out.
Nomex is manufactured in many different forms for many uses such as circuit boards, electrical wire insulation, air filters, carpet, upholstry, insulating blankets, paper, etc... I doubt fiber shedding is an issue...
Keep in mind Nomex is not fireproof, it will burn with direct flame impingment, it will not however support combustion. From what I've been able to find it can sustain temps of about 400F without problems. This "should" be adequate for your application.
If it were me, I would lean toward the ceramic insulation I use when working on chimneys. It is designed to wrap around a stainless steel liner and withstand solid fuel chimney temperatures. 1/2" of this insulation and you can lay your hand on the outside of an 700F chimney without harm.
HTH
Tim
